F(x, y) = y2 + (x·y + 1)3 = 0
Fx'(x, y) = 3·x2·y3 + 6·x·y2 + 3·y
Fy'(x, y) = 3·x3·y2 + 6·x2·y + 3·x + 2·y
y'(x, y) = -Fx'(x, y) / Fy'(x, y)
y'(x, y) = -(3·x2·y3 + 6·x·y2 + 3·y) / (3·x3·y2 + 6·x2·y + 3·x + 2·y)
y'(2, -1) = -(3·22·(-1)3 + 6·2·(-1)2 + 3·(-1)) / (3·23·(-1)2 + 6·22·(-1) + 3·2 + 2·(-1)) = 0.75
y = 0.75·(x - 2) - 1